A water-soluble esipt fluorescent probe with high quantum yield and red emission for ratiometric detection of inorganic and organic palladium

Article abstract of DOI:10.1002/asia.201500114

A novel fluorescent probe with a high quantum yield (0.41), large Stokes shifts (255 nm), and red emission (635 nm) was designed to detect all typical oxidation states of palladium species (0, +2, +4) by palladium-mediated terminal propargyl ethers cleavage reaction in water solution without any organic media. The probe showed a high selectivity and excellent sensitivity for palladium species, with a detection as low as 57 nM (6.2 μgL^-1).

Full text of DOI:10.1002/asia.201500114

CHEMISTRY
AN ASIAN JOURNAL
Accepted Article
Title: A novel water-soluble ESIPT fluorescent probe with high quantum yield and
red emission for ratiometric detection to inorganic and organic palladium
Authors: Tang Gao; Pengfei Xu; Meihui Liu; Anyao Bi; Wenbin Zeng
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To be cited as: Chem. Asian J. 10.1002/asia.201500114
Link to VoR: http://dx.doi.org/10.1002/asia.201500114
A sister journal of Angewandte Chemie
and Chemistry – A European Journal
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Chemistry - An Asian Journal
10.1002/asia.201500114
COMMUNICATION
A novel water-soluble ESIPT fluorescent probe with high
quantum yield and red emission for ratiometric detection to
inorganic andorganic palladium
Tang Gao‡,1, Pengfei Xu‡,1, Meihui Liu1, Anyao Bi1, Pengzhi Hu2, Bin Ye2, Wei Wang2, Wenbin Zeng*,1
Abstract: A novel fluorescent probe with high quantum yield (0.41),
Among the reports probe based on ESIPT(excited-state intra-
molecular proton transfer), 2-(2-hydroxyphenyl) benzothiazole
large stokes shifts (255 nm) and red emission (635 nm) was
designed to detect all typical oxidation states of palladium species (0, (HBT) and 2-(2-hydroxyphenyl) benzoxazole (HBO) are the most
+2, +4) by palladium-mediated terminal propargyl ethers cleavage
reaction in water solution without any organic media assistance. The
attractive showing remarkable results with selectivity and
sensitivity for detection of H2S,9 Hg2+,10 phosphatase MKP-6,11
Co2+,12superoxide13 and Pd2+.7j However, there are still some
limitations that should be noticed such as quantum yields of
HBT is low to 0.00514and the emission color of HBT and HBO
derivatives are usually limit in the blue and green region
(λemmax< 600 nm), with a few examples in the red and near-
infrared region.15,7j Furthermore, in protic/polar solvents, the keto
emission of HBT often diminish, because of the inhibiting of the
proton transfer in the excited state.15a All these drawbacks
severely inhibit their practical application in biosensing
andbioimaging16, and promote us to tune the emission to longer
wavelength while retaining the ESIPT process optimal in
aqueous solution.
In our strategy, an electron acceptor (1,4-dimethylpyridin-1-
ium iodide) in the ortho-position of phenol is introduced to
extend the π-electron system. Also the picoline salt groups
impart good water solubility to the probe. On the other hand, in
ESIPT process upon photo excitation, HBT can undergo fast
proton transfer reaction in a sub picosecond time scale and
imino nitrogen is converted into an amine group which can serve
as an electron donor group.16 It is logical to expect that an
intramolecular charge transfer (ICT) coupled ESIPT process in
HBTP may lead to dramatically fluorescent property change
involving red shift emission and large Stokes shift. Furthermore,
modification of the hydroxyl group of HBTP by recognition group
can block the ESIPT process and result in the enol emission.17
While, regeneration of the free hydroxyl group can restore the
keto emission. Following the described strategy above,
propargyl ether as a functional group was chosen since it can
react with palladium species.18 Probe HBTPQ was synthesized
according to the synthetic pathway as shown in Scheme 1.
Compound 2 was prepared by the modified Duff reaction from 1.
The intermediate HBTP was synthesized by Knoevenagel
probe showed
a high selectivity and excellent sensitivity for
palladium species, with a detection as low as 57 nM (6.2 μg/L).
As a rare inner transition metal, palladium has been widely
used in various functional materials, such as dental crowns, fuel
cells, jewelry, and especially catalysts in synthesizing various
drugs because of its excellent catalytic ability.1 However, the
residual palladium in the resultant compounds is typically at a
level of 300–2000 ppm,2 drastically higher than governmental
restrictions on the threshold for palladium in drugs (5–10 ppm).3
It is well known that palladium species can form complexes with
some biomacromolecules such as thiol-containing amino acids,
protein, DNA, RNA, and vitamins, and thus may disturb many
biological processes and finally become a health hazard.4
Therefore, it is necessary and important to develop effective and
convenient methods for detecting palladium species, especially
in living systems and the environment settings . During the past
decades, some conventional analytical methods for palladium
species have been developed, such as atomic absorption
spectrometry (AAS), inductively coupled plasma mass
spectrometry (ICP-MS), solid phase micro extraction high
performance liquid chromatography, X-ray fluorescence.5
However, these methods usually require sophisticated and time-
consuming sample preparation procedures or expensive
equipment. In contrast, optical detection methods, particularly
fluorescence methods have attracted considerable attention6
due to the convenient operation, low cost, high sensitivity and
selectivity.
Many efforts have put into developing fluorescent methods to
detect palladium7. However, there are still some limitations for
the quantitative detection. For example, though there are few
sensors for detection to palladium in water,7ialmost all probes
have been reported for palladium only work in organic solvents
or water with organic media, which limit their practical
condensation reaction using compound
2
and 1,4-
dimethylpyridin-1-ium iodide as reactants. Reaction of HBTP
with 3-bromoprop-1-yne afforded the probe HBTPQ. The
application
in
biological
imaging
or
environmental
1
structure of HBTPQ was characterized by H NMR, 13C NMR,
monitoring.7Further, water-solubility remains a prerequisite for
their use in relevant biosensing and bioimaging applications.
Chemosensors for palladium are based on the fluorescein and
rhodamine fluorophores, possessing small stokes shifts and give
emission in the visible region, thus may have potential difficulties
in the quantitative determination and bioimaging due to the
excitation interference and photo-damage.7b-7e Moreover , these
on−off and off−on probes are significantly influenced by the
environmental effects and excited energy,7f-7h while the
ratiometric probe can avoid interference from factors such as the
probe concentration, photobleaching, instrument’s sensitivity,
and environmental effects.8 With these considerations in mind,
we set out to develop a ratiometric fluorescent probe for
palladium species that would function in an absolute aqueous
medium with large stokes shifts and long wavelength emission.
and HRMS.
Scheme 1.Synthesis of probe HBTPQ and sensing mechanism.
With HBTP in hand, we explore the keto emission of HBTP in
protic/polar solvents. As shown in Fig. S1a, HBTP exhibited a
strong keto emission (maximized at > 600 nm) with large Stokes
shifts in normal strong polar and protic solvents (DMF, DMSO,
CH3CN, ethanol and H2O) at low concentration and a high quan-
T. Gao‡, P. Xu‡, M. Liu, A. Bi, P. Hu, B. Ye, Prof. W. Wang, Prof. W. Zeng*
‡These authors contributed equally
1School of Pharmaceutical Sciences, 2The third Xiangya Hospital, Central
South University, Changsha410013, China.
Email: wbzeng@hotmail.com

Chemistry - An Asian Journal
10.1002/asia.201500114
COMMUNICATION
tum yield up to 0.41 was achieved in CH3CN. Absorption
spectrum of HBTP in the different solvents was provided in Fig.
S1b.
the fluorescence change is owing to the formation of compound
HBTP.
The fluorescence sensing behavior of HBTPQ towards
palladium was investigated using a 10 μM solution of HBTPQ in
buffered (10 mM PBS, pH = 7.4). PdCl2 was selected as the
representative palladium species in the following titration
experiments since it is most toxic of palladium compounds. To
determine the response time of probe HBTPQ for Pd, the time-
dependent fluorescence intensity changes of HBTPQ was tested.
The response time of the reaction system is about 30 min at
37 °C (Fig. S2).
The spectroscopic properties of the compound HBTPQ (10
μM) were measured in PBS (10 mM, pH = 7.4).For the
application in an approximate physiological condition, we carried
out the experiments at 37 oC. As shown in Fig. 1, probe HBTPQ
exhibits a major absorption and emission peak at 330 nm and
495 nm, respectively. As expected, compound HBTPQ only
display the enol emission at 495 nm when excited at 380 nm,
because of preventing the excited state intramolecular proton
transfer process by the protection group. Upon the addition of
Pd2+ (30 μM), the initial absorption peak centered at 325 nm
decreased gradually and a new band appeared at 380 nm while
the maximum emission peak showed up at 635 nm with the
large stokes shift about 255 nm owing to the process of ESIPT
upon photoexcitation. Additionally, the marked color changes
were also noticed. The obvious changes in both the absorption
and fluorescence spectra indicated that a free HBTP was
generated by the palladium-triggered cleavage reaction.
Upon PdCl2 was added gradually to the solution of HBTPQ,
the absorption peak at 325 nm decreased gradually and the
absorption at 380nm increased with an isosbestic point at 365
nm (Fig. S3). As shown in Fig. 2, when the probe HBTPQ was
treated with different concentrations of PdCl2 (0−50 μM) the
emission band at 495 nm decreased progressively, while a new
red-shifted emission peak was observed around 635 nm with an
isoemission point at 550 nm. Importantly, the ratio of emission
intensities (I635nm/I495nm
) increased by nearly 30-fold and
exhibited good linear correlation with the amount of Pd2+ (0−10
μM, R2 = 0.992) (Fig. 2b) and a maximal fluorescent ratio signal
is achieved in the presence of only 3 equiv of Pd2+(Fig. 2a inset).
The detection limit of probe 1 for Pd2+ was determined as 57 nM
(Pd2+ con. = 6.2 μg/L), which was far below the restricted level of
palladium ions (5–10 ppm) from the European Agency for the
Evaluation of Medicinal Products (EMEA).3 This result indicated
that probe 1 can be a sensitive ratiometric fluorescent sensor for
quantitative detection of Pd2+ in the aqueous environments.
Fig. 1 (a) UV−vis absorption spectra of probe HBTPQ (10 μM) in the absence
and presence of Pd2+ (30 μM). Inset: Color changes in probe HBTPQ upon
addition of Pd2+. (b) Fluorescence spectra of probe HBTPQ (10 μM) in the
absence and presence of Pd2+ (30 μM). Inset: Fluorescence changes excited
by UV lamp (365 nm) in probe 1 upon addition of Pd2+. All spectra were
acquired 1 h after Pd2+ addition at 37 oC. Slit: 3.0 nm/3.0 nm.
To verify the sensing mechanism of HBTPQ toward palladium
species, the reaction of HBTPQ with Pd2+ was conducted under
the same conditions as described above. The reaction product
was subjected to HRMS analysis. The peak at m/z 345.1052
[M+] was observed in the HMRS spectra of the product mixture
corresponding to compound HBTP (ESI). Therefore, this
recognition is implemented by the depropargylation reaction and
Fig. 2 (a) Fluorescence responses of probe HBTPQ (10 μM) toward different
concentrations of Pd2+(0
−
50 μM). Inset:Ratiometric calibration curve
I635nm/I495nm as a function of Pd2+. (b) Fluorescence intensity ratio (I635nm/I495nm
of probe HBTPQversus increasing concentrations of Pd2+. All spectra we are
)

Chemistry - An Asian Journal
10.1002/asia.201500114
COMMUNICATION
acquired 0.5 h after Pd2+ addition at 37 °C in PBS buffer solutions (10 mM, pH
= 7.4). Ex = 380 nm. Slit: 3.0 nm/3.0 nm
system showed a noticeable emission ratio (I635 nm/I495 nm) change
within the pH range of 6−9. Thus, this probe could be used to
detect palladium species in physiological conditions without
interference.
Next, to test whether probe HBTPQ could detect all the
different initial oxidation states of palladium such as [Pd(0):
Pd(PPh3)4; Pd(II): PdCl2, Pd(OAc)2; Pd(IV): (NH4)2PdCl6]. To our
delight, the results reveal that chemosensor HBTPQ remarkably
responds to all the oxidation states of palladium with significant
fluorescence enhancement without additional reagents (Fig. 3).
The proposed mechanism of reaction is shown in Scheme S1.In
the presence of Pd(0) alleny lpalladium intermediate was form
through an oxidative addition of Pd(0) into the alkyne C–H bond.
The allenyl palladium intermediate then undergoes nucleophilic
attack on the central sp carbon by a water molecule then
releasing the desired hydroxyl of Phenol.19The hydration of a
propargyl ether catalyzed by Pd(II)/Pd(IV) species to form an
internal and a terminal carbonyl compound, the latter of which
can through β-elimination (depropargylation).20
In conclusion, we have developed a novel ESIPT fluorescent
dye HBTP with high quantum yield (0.41) and red emission (635
nm). HBTP exhibited strong keto emissions with large Stokes
shifts in normal strong polar and protic solvents. By virtue of
HBTP, we constructed a ratiometric and colorimetric fluorescent
sensor HBTPQ, which is specific for palladium species based on
the palladium-mediated propargyl ether chemistry. The results
demonstrate that sensor HBTPQ exhibits a high sensitivity,
excellent selectivity and low detection limit (57 nM) for Pd2+ in
aqueous solution. Moreover, obvious changes of emission ratio
(I635nm/I495nm) were observed towards all the oxidation states of
palladium (0, +2, +4) without additional reagents. This work pro-
vides a new promising strategy for researchers to explore
fluorescent ratiometric probes based ESIPT process.
Acknowledgements
This research was supported in part by China (81271634) and
Hunan Provincial Natural Science Foundation of China
(12JJ1012).
Keywords: water-soluble; excited state intramolecular proton
transfer; palladium detection; ratiometric fluorescent probe; red
emission
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Chemistry - An Asian Journal
10.1002/asia.201500114
COMMUNICATION
Tang Gao‡,1, Pengfei Xu‡,1, Meihui Liu1,
Anyao Bi1, Pengzhi Hu2, Bin Ye2, Wei
Wang2, Wenbin Zeng* 1
Page No. – Page No.
A novel water-soluble ESIPT
fluorescent probe with high quantum
yield and red emission for ratiometric
detection to inorganic and organic
palladium
We developed the first novel fluorescent probe with high quantum yield
(0.41), large stokes shifts (255 nm) and red emission (635 nm) was designed
to detect inorganic and organic palladium species in water.